The invention relates to an apparatus for writing patterns in a layer on a substrate by means of a beam of electrically charged particles, comprising a source for generating the beam, a charged-particle lens system for focussing the beam on the layer, and means for moving the beam and the substrate relative to each other in directions transverse to the beam axis.
Such apparatus may for example be used to expose certain areas of a layer of electron-sensitive resist to an electron beam, so that either these areas or the rest of the layer may subsequently be etched away. Other applications are the removal of selected areas on a metal layer, or ion-implantation in selected areas of a semiconductor layer. This type of technique is for example employed in the manufacture of so-called micro-circuits. These micro-circuits may be magnetic-domain memories ("bubble memories") or integrated electronic circuits (hereinafter abbreviated I.C., from "integrated circuit"). The resolution that can be achieved with a beam of charged particles, which is generally an electron beam but may alternatively be an ion beam, is greater than the resolution that can be achieved when a beam of electromagnetic radiation, for example ultraviolet light, is used.
In the article: "An Electron Beam Maskmaker" in: I.E.E.E. Transactions on Electron Devices", Vol. ED-22, No. 7, July 1975, pages 376-384, a so-called "electron beam writer" is described, i.e. a device in which a layer of a substrate is exposed to an electron beam in accordance with a specific pattern. By means of such a device microcircuit masks can be manufactured, with the aid of which masks the desired I.C. patterns of magnetic-domain patterns can be projected on a layer of a substrate, such as a slice of silicon. Alternatively, such a writer enables microcircuit patterns to be written directly into the layer.
Apart from the higher resolution, the use of an electron beam in comparison with the use of an optical beam in the manufacture of microcircuits, has some further advantages, namely a greater flexibility and the possibility of writing extensive patterns with very small details. In the case of direct writing in the layer on the substrate of the microcircuits with a charged-particle beam, a number of steps in the manufacturing process can be avoided, so that the risk of errors is reduced.
In a device for writing with a beam of charged particles the beam should be positioned very accurately on the layer to be inscribed. This positioning is effected by adjusting the angle .phi. between the beam and the axis of the charged-particle lens system. In order to obtain the desired positioning accuracy, the height of the layer to be inscribed relative to the lens system would have to remain constant with great accuracy. It has been found that in practice there are always variations in the height because of inaccuracies in the mechanical guidance of the substrate relative to the lens system, unevenesses of the substrate, and replacement of the substrate in the writing apparatus after the substrate has been removed therefrom for the performance of lithographic process steps. Therefore, it is necessary that the position of the layer to be inscribed relative to the lens system be measured, so as to enable the deflection angle .phi. to be adjusted on the basis of the measurement.
As is described in the article "An Electron Beam Maskmaker", additional markers may be provided on the substrate for this purpose, for example in the form of small squares of a heavy metal such as gold. Before the desired pattern is written the markers are scanned by the beam and the electron backscatter on the markers is measured. Thus the actual positions of the markers can be determined, so that the deviations in the height of the layer to be inscribed and the positioning of the beam are known for areas around the markers. This height measurement has the disadvantages that additional process steps are necessary, namely depositing the markers on the substrate and scanning the substrate before inscribing, and that by providing the markers the electrical behavior of the semiconductor circuit to be manufactured may be affected.
It could also be considered to make the charged-particle lens system telecentric, i.e. to add an extra lens to this system, so that for any position in the layer to be inscribed the angle .phi. between the beam and the axis of the lens system becomes zero. The addition of an extra lens, however, complicates the writing apparatus. Making the lens system telecentric moreover means that a compromise must be made with respect to certain parameters of this system, such as the image field or the angle of aperture.